1. Field of the Invention
The invention relates to a light source device using a high pressure mercury discharge lamp which is used, for example, as the light source for an optical device, such as a projector or the like, or using a discharge lamp with high radiance (HID lamp), such as a metal halide lamp or the like.
2. Description of Related Art
In a light source device for an optical device, such as a liquid crystal projector, a DLP™ projector (Texas Instruments) or the like, a discharge lamp with high radiance is used.
In the case of operating this type of discharge lamp, in the state where a voltage is applied to the lamp, which is called the no-load voltage, the no-load voltage is combined with a high voltage, by which an insulation breakdown is produced in the discharge space, and a transition from a glow discharge to an arc discharge takes place. The relation between the lamp voltage (VL) and the lamp current (IL) after the transition into the arc discharge is described below in relation to FIG. 7.
Directly after the transition to an arc discharge the lamp voltage is a low voltage of for example roughly 10 V. If the attempt is made to supply a nominal wattage to the lamp starting at this instant, an overly large current must be allowed to flow. Normally, therefore, as is shown using the point (P01) in FIG. 7, the circuit of a feed device is controlled such that only one lamp current flows wherein the current is smaller than the current boundary value (IL0) of a certain value. The lamp voltage, which is low immediately after the transition to an arc discharge, increases according to the temperature increase of the lamp, and causes a transition in the manner shown using the arrow (Y01) until the nominal wattage can be supplied to the lamp by a lamp current within the range of the current boundary value (IL0) in the manner shown using point (P02).
In FIG. 7, the constant wattage characteristic (Fps0) shows the condition under which the wattage to be supplied to the lamp becomes identical to the nominal wattage, i.e., the condition under which the product of the lamp voltage (VL) and the lamp current (IL) becomes constant. In the range where the lamp current is less than the current boundary value (IL0), the circuit of the feed device is controlled such that the ratio between the lamp voltage (VL) and the lamp current (IL) is essentially above the constant wattage characteristic (Fp0).
If the lamp temperature continues to increase, along the constant wattage characteristic (Fp0) a transition takes place, as shown using the arrow (Y02), until finally the increase of the lamp voltage does not proceed any farther, by which the state of a saturation voltage shown using the point (P03) is reached.
Also, the current boundary value (IL0) is inherently a high current. Since the heat generation of the power elements, such as the switching devices, such as FETs or the like, comprising the feed device, semiconductor elements such as diodes or like, coils like reactance coils or the like and similar elements, is large, the feed device may not be able to withstand a constant flow of this lamp current. However, since, as described above, the flow of this high current is limited to a short time until the lamp voltage rises and is saturated, it is normally not considered disadvantageous. Since, on the other hand, the lamp current which flows after reaching the saturation voltage is of a constant nature, there is a disadvantage if it is large, as in the case shown using the point (P03′). This condition arises in a lamp with a low saturation lamp voltage.
Since recently the degree of utilization of the light emitted from the discharge lamp has been increasing more, the more the light source has approached a point light source, there is more and more a demand for shortening the distance between the electrodes of the discharge lamp in order to reduce the size of the emission area of the discharge lamp. However, if, as described above, the distance between the electrodes is shortened, the amount of influence exerted by the deviation from the normal value of the electrode distance which arises due to processing inaccuracies in lamp manufacture, as a result of thermal expansion during lamp operation, as a result of the transport phenomenon of the electrode material and the like, on the saturation voltage of the above described lamp voltage, becomes stronger than in the conventional case of a long distance between the electrodes.
The reason for this is as follows:
In a lamp where the normal value of the electrode distance is 2 mm, the variance in a processing error of for example ±0.2 mm is at most ±10%. However, in a lamp in which the normal value of the electrode distance is 0.8 mm, the variance is ±5%. Since the saturation lamp voltage is essentially proportional to the dimension of the distance between the electrodes, the percentage of this variance of dimension is unchanged to the percentage of variance of the saturation lamp voltage.
If it is assumed that the parameters of the lamp, such as the fill pressure during operation, for example, is set such that in a lamp with a normal value of the electrode distance of 2 mm and in a lamp with a normal value of the electrode distance of 0.8 mm the saturation lamp voltage in the case in which the dimension of the distance between the electrodes has a normal value, i.e. the normal lamp voltage, reaches the same value, for example, 70 V, the lamp voltage under the condition under which the lamp voltage has the minimum value, i.e., in the case of an error of the electrode distance of −0.2 mm in the lamp with the normal value of the electrode distance of 2 mm is 63 V and for a lamp with the normal value of the electrode distance of 0.8 mm, is 52.5 V.
As described above, in one such light source device, the feed device is made such that the wattage to be supplied to the lamp is also essentially constant when the lamp voltage changes. The lower the lamp voltage, the greater the lamp current which must be allowed to flow. If it is assumed, for example, that the nominal wattage of the lamps with the normal values of the electrode distance of 2 mm and 0.8 mm is the same value, for example, 200 W, the respective maximum current in the lamp with the normal value of the electrode distance of 2 mm (with a minimum lamp voltage of 63 V) is 3.17 A and in the lamp with the normal value of 0.8 mm (with a minimum lamp voltage of 52.5 V) it is 3.81 V. If the construction of the lamp is fixed, from the standpoint of reliable and at the same time stable use of the lamp, the nominal lamp wattage is also determined. With respect to a condition with such great variances there is however neither the nominal lamp voltage nor the nominal lamp current.
The loss of the power elements comprising the feed device generally increases in proportion to the square of the flowing current. If the loss of the feed device during operation of the above described lamp with a normal value of the electrode distance of 2 mm at a minimum voltage, i.e., maximum loss, is for example 10 W, in the case of a lamp with a normal value of the electrode distance of 0.8 mm, the maximum loss is roughly 14 W.
In the case in which the normal value of the electrode distance of 2 mm is reduced to 0.8 mm, it becomes apparent that, as a result of a processing fault in the electrode distance, the maximum lamp current increases and that the maximum loss increases by 44%, even if the lamp is built such that the normal lamp voltage does not change. There is, therefore, the disadvantage that the heat generation of the above described power elements comprising the feed device increases. To reduce this disadvantage, it is necessary to use larger switching devices, diodes, coils and the like, to increase the size of the cooling rib or to reinforce it. Hence, an increase in size and weight of the optical device and an increase in cost become inevitable.
To eliminate this disadvantage, a method is devised in which even in a lamp with a low saturation lamp voltage the above described current boundary value (IL0) is set to be small, wherein the amount at which heat generation of the above described power elements is not regarded as disadvantageous. In this case there, however, the disadvantage of the time consumption for start-up to reaching practical radiance of the lamp is great. Furthermore, this measure resulted in the disadvantage that the phenomenon often occurs where the radiance spot of the arc discharge is not stabilized such that flickering occurs. There is specifically the disadvantage that so-called flickering often forms. The reason for this is that, due to this measure, a transition to stable operation takes place when the lamp is not adequately heated up at the nominal wattage, and that the vaporization of the substance filling the lamp, for example mercury or the like, is incomplete. This measure can, therefore, not be regarded as a genuine solution.
Japanese patent 3261142 describes a device for operating a discharge lamp in which use of a winding with a large winding diameter such as a coil or the like can be avoided. The following measures are used in the reference:                A time is set by a timer circuit which is necessary for the lamp voltage to reach essentially the nominal lamp voltage at least immediately after starting of the discharge lamp with high radiance.        Until this time expires, the time up to a stable amount of light is shortened by high current flow as the nominal lamp current.        In the case after this time expires the lamp voltage does not reach the nominal voltage, a DC-DC converter circuit of the voltage increasing chopper type is controlled such that a current smaller than the lamp current which can maintain the nominal operating state flows.        
In this device for operating a discharge lamp, the time necessary for the lamp voltage to reach essentially the nominal lamp voltage is set by the timer circuit. In the above described case in which the dimension of the distance between the electrodes of the lamp inherently has variances, in the lamp characteristic and in the time behavior of the lamp, there are variances due to the fact that variances of the amount of filler substance, such as mercury or the like; and, variances of the cooling conditions of the optical device and the ambient temperature are added to the above described variances. Thus, for the time in which the nominal wattage is to be supplied, major variances arise. Therefore, the time in which a high lamp current is intentionally allowed to flow cannot be determined beforehand. For example, the degree of the wattage which exceeds an allowable value was too great, resulting in the danger that the lamp service life is shortened or breakage occurs.
The object of this device for operating a discharge lamp is to exactly distinguish from one another a normal lamp and a lamp in which there has been a slow leak and to prevent an overly large current from continuing to flow in a lamp in which there was a slow leak and in which a fault has occurred. Therefore, this device is not used to normally operate a lamp which is indeed normal but which has great variances, and, therefore, a high current is conveyed, in a manner which corresponds to these variances to make it usable.